A track-and-hold (T/H) circuit tracks an analog input signal and, at specified intervals, holds the amplitude of the signal, which is then sampled for subsequent use. Consequently, the T/H circuit operates by repeatedly switching between a track mode of operation and a hold mode of operation. A typical T/H circuit includes a hold capacitor that stores an electrical charge during a track mode of operation and holds the stored electrical charge during a hold mode of operation. The electrical charge stored in the hold capacitor is held by electrically isolating the hold capacitor when the T/H circuit is switched from the track mode to the hold mode. The electrical charge held by the hold capacitor reflects the amplitude of the input analog signal at the instant when the operational mode of the T/H circuit was changed from track to hold. The held value is then sampled and used, for example, as an input signal for an analog-to-digital (AD) converter.
One of the most basic T/H circuits includes a switch connected between an input node and an output node, and a shunt hold capacitor connected between the switch and the output node. During a track mode of operation, the switch is closed, allowing the hold capacitor to charge due to an analog input signal applied to the input node. During a hold mode of operation, the switch is opened, isolating the hold capacitor from the input analog signal. Thus, the electrical charge of the hold capacitor is held at the state when the switch was opened, which can be sampled through the output node. Although this basic T/H circuit can be used effectively in certain applications, a T/H circuit having better performance with respect to signal feedthrough, bandwidth, and switching speed is needed for use in, for example, high-speed analog-to-digital converters.
A conventional T/H circuit of interest that can be used in high-speed analog-to-digital converters is a switched emitter follower T/H circuit. A switched emitter follower T/H circuit utilizes an emitter follower to selectively charge a hold capacitor. The input analog signal is applied to the base of the emitter follower to track the signal during a track mode. The switched emitter follower T/H circuit includes an off resistor on the input signal path to ensure that the emitter follower is turned off during a hold mode. The switched emitter follower T/H circuit also includes a cancellation capacitor connected to the base of the emitter follower to reduce the input signal feeding through the emitter follower to the hold capacitor during a hold mode.
A concern with the switched emitter follower T/H circuit is that it is difficult to effectively cancel signal feedthrough with the cancellation capacitor since the signal feedthrough is caused by voltage dependent parasitic capacitance (base emitter capacitance). Another concern with the switched emitter follower T/H circuit is that the off resistor on the input signal path reduces the achievable bandwidth of the T/H circuit since the off resistor and the input capacitance of the emitter follower act like a RC-lowpass filter. This is further exacerbated by the cancellation capacitor, which increases the total input capacitance of the T/H circuit.
In view of these concerns, what is needed is a circuit and method for performing track and hold operations with wider bandwidth, higher signal-to-noise ratio and greater switching speed than comparable conventional T/H circuits.